ACGIH concluded that non-fibrous forms of SiC have a very low toxicity in humans and experimental animals. The threshold limit value – time-weighted average (TLV-TWA) for non-fibrous forms was, therefore, set at 10 mg/m3 for
inhalable particles and 3 mg/m3 for respirable particles.
As regards SiC fibres, ACGIH stated that the human clinical and epidemiologic data, combined with animal experimental data, indicate that SiC fibres can cause lung fibrosis, cancer and possibly mesothelioma. It was further stated that animal data indicate that the toxicity of SiC fibres is similar to crocidolite asbestos on a fibre-to-fibre basis. Thus, the health risks from exposures to SiC fibres should be considered of the same magnitude as those caused by crocidolite asbestos and exposure to airborne SiC fibres should therefore be limited to the level set for crocidolite asbestos, namely, a TLV-TWA of 0.1 fibre/cm3. The carcinogen
designation of A2, Suspected Human Carcinogen, was recommended for fibrous forms.
There were no data upon which to base skin or sensitiser (SEN) notations or recommend a TLV-STEL (short-term exposure limit) (1).
German Research Foundation (DFG) 1997, fibrous SiC
The MAK Commission referred to data showing that two samples of SiC whiskers produced marked and dose-dependent increases in the tumour incidence in rats after administration by the intratracheal, intraperitoneal or intrapleural routes. The carcinogenic potency of SiC fibres appeared to be high since increased tumour incidences were produced even by small numbers of critical fibres. It was further stated that one of these samples showed cell transforming activity in vitro in mesothelial cells from the rat diaphragm and pulmonary epithelial cells from the hamster.
It was concluded that the studies with intratracheal, intraperitoneal or intra- pleural administration revealed clear evidence of a marked carcinogenic activity of SiC fibres in experimental animals and that these results were in accordance with the cell transforming activity demonstrated in vitro.
SiC fibrous dust was therefore to be handled like those fibrous dusts classified as “Substances that are considered to be carcinogenic for man because sufficient
data from long-term animal studies or limited evidence from animal studies substantiated by evidence from epidemiological studies indicate that they can contribute to cancer risk. Limited data from animal studies can be supported by evidence that the substance causes cancer by a mode of action that is relevant to man and by results of in vitro tests and short-term animal studies” (category IIIA2, i.e. present category 2) (47).
German Research Foundation (DFG) 2013, fibre-free SiC
The MAK Commission regarded fibre-free SiC to be a granular biopersistent dust. Based on the available data, in particular the in vitro data, it was not considered clear whether SiC dust has substance-specific effects beyond the general particle effects. Several studies among employees in the SiC industry indicate a fibrotic effect of SiC particles.
The present epidemiological studies on workers in the SiC production were regarded unsuitable for assessing the carcinogenic effects of fibre-free SiC because exposure was to a mixture of fibres and particles.
In a rat carcinogenicity study by intraperitoneal application, particulate SiC induced no mesotheliomas. Data to assess the carcinogenic effects were considered limited.
Overall, it was concluded that human experience and animal studies that are appropriate for deriving a MAK value were not present and an assignment to a maximum limitation category omitted.
It was further stated that SiC is water insoluble and dermal absorption therefore unlikely. Thus, fibre-free SiC was not marked with “H”.
No data were available for evaluation of sensitisation or teratogenicity (48).
Health Council of the Netherlands: Evaluation of the carcinogenicity and genotoxicity 2012
The Committee concluded that fibrous SiC (fibres, whiskers) may cause cancer according to a non-stochastic mechanism and should be classified as “carcino- genic to humans” (category 1A). The data on the angular form of SiC were considered insufficient to classify the carcinogenic properties of this substance (category 3). The Committee was concerned about the question whether the commercial angular SiC is sufficiently free of fibrous SiC (75).
International Agency for Research on Cancer (IARC) 2017, SiC whiskers and fibres
Occupational exposures associated with the Acheson process were classified as carcinogenic to humans (Group 1) on the basis of sufficient evidence in humans that they cause lung cancer. Since the correlation between exposures to SiC fibres and cristobalite made it difficult to disentangle their independent effects, the Working Group concluded that fibrous SiC is possibly carcinogenic to humans (Group 2B) based on limited evidence in humans that it causes lung cancer. No data on cancer in humans exposed to SiC whiskers were available. In experimental animals, there was sufficient evidence for the carcinogenicity of SiC whiskers.
Although not unanimous, the Working Group classified SiC whiskers as probably carcinogenic to humans (Group 2A). It was stated that SiC whiskers are mono- crystalline and homogeneous in form, while fibrous SiC is mostly polycrystalline and heterogeneous in form. The physico-chemical characteristics of these fibres were considered to be distinct. The majority of the Working Group therefore considered that the differences in the nature of SiC fibres and SiC whiskers warranted separate evaluations (78).